The invention relates to polyimide foams and in particular to a method of and apparatus for continuously foaming a polyimide prepolymer powder that is not susceptible to microwave heating wherein the polyimide prepolymer powder is uniformly heated throughout to form a continuous piece of foam product of relatively uniform cell size, density and strength throughout.
Many foams and especially polyimide foams are produced by batch processes where the prepolymer powder is foamed in a closed or open mold or by the free rise method. As discussed in U.S. Pat. No. 4,780,167, (columns 1 and 2), the foam produced in closed molds tends to be very irregular and lack uniformity of cell size, density and strength. The problems increase when attempts are made to vary the foam product density by varying the amount of precursor placed in the mold. While foaming product under ambient pressure in a free rise process or in open molds produces more uniform cell size and density, it is still difficult to produce foam panels of controlled, varying densities for different applications. In addition, when using batch operations to foam polyimides, it is necessary to use masking materials or release materials to prevent the foam from adhering to the mold contact surfaces and/or the mold contact surfaces must be cleaned between batches. In a free rise batch process as discussed in U.S. Pat. No. 4,305,796, (column 5, line 51), the faces and all four sides of the buns produced in the batch process normally must be trimmed to provide the end product with smooth planer surfaces. This results in trim or scrap losses which increase the cost of the end product.
The '167 patent further discusses the foaming of certain resins between moving endless belts. The patent states that while this method of foaming is apparently effective with polystyrene and polyurethane "this method is not applicable to controlled density polyimide foam because of the much longer periods at much higher temperatures required for polyimides which require excessively long and/or very slowly moving belts operating in a very high temperature oven."
In addition, the heat transfer situation is very different for a polyimide foam when compared to phenolic and polyurethane foams which are formed from ingredients that are much more reactive than the ingredients used to produce polyimide foam. With the phenolic and polyurethane foams, the reactive ingredients are mixed together in a mixing head and are deposited within seconds onto the conveyor where the ingredients react and produce heat. The blowing agent is a volatile compound (such as a chlorofluorocarbon) that has been added to the mixture specifically for blowing. In this process, the oven's heat is used to raise and/or keep the temperatures of the outer surfaces of the foam at the same temperatures as those generated within the foam during the foaming process due to the exotherm of the reacting ingredients.
With polyimide foam, the ingredients react more slowly and the heat from the oven is required to promote the reaction of the ingredients throughout the foam. As the ingredients in the upper portion of the polyimide powder are foamed, the upper foamed portion insulates the lower portions of the ingredients from the hot oven air or infrared radiation thereby retarding the foaming process and producing nonuniform temperatures throughout the thickness of the ingredients being foamed.
U.S. Pat. No. 4,900,761, is directed to a process for producing a polyimide foam wherein "In at least one such stage the precursor is subjected to one or more temperatures sufficient to obtain a consolidated but friable cellular foam structure, and in at least one other such stage this cellular foam structure is subjected to one or more higher temperatures sufficient to cure the cellular material into a resilient polyimide foam. Preferably, these stages are conducted in a continuous manner as by supporting the material being foamed on a moving belt or rotating platform associated with appropriate heating apparatus...". While directed to a continuous process of foaming, the process differs in its approach from the method of the present invention in certain important respects and does not address the need for uniform heating of the ingredients throughout the entire thickness of the materials being foamed by balancing the heat imparted to the foam from above and below by preheating the moving belt. In addition, it does not address the need to rapidly cool the lower surface of the foam product formed so that the foam can be removed from the moving belt before the interior of the foam is completely cool to increase the efficiency of the continuous foaming process.
U.S. Pat. No. 4,855,331, is directed to the production of foamed polymer structures in a batch process which uses microwave radiation to foam precursors that are susceptible to microwave heating. In this batch process microwave radiation is applied to the precursor from above, the sides and the ends of the precursor body. However a shield is placed below the precursor body to curtail radiation directed at the precursor body from below. The shield can be heated to a temperature of between about 50 degrees and about 200 degrees Centigrade (preferably in the range of about 60 degrees to about 130 degrees Centigrade) to raise the temperature of at least the lower portion of the precursor body.
U.S. Pat. No. 4,305,796, is directed to a method of preparing polyimides from precursors which are susceptible to microwave heating in a batch process using microwave radiation. In this batch process the substrate or mold can be preheated to a temperature of about 121 degrees Centigrade to 149 degrees Centigrade.